Electrospray ionization refers to a method of providing ionized molecules from a liquid sample. The electrospray ionization process generates highly-charged droplets from the liquid sample. As solvent evaporates from the droplets, gas phase ions representative of the species contained in the liquid sample are generated. The ions are then introduced into an analyzer (e.g. a mass spectrometer) via an ion-sampling interface coupled to the analyzer. FIGS. 1A and 1B illustrate examples of a conventional electrospray ion source 102a and an orthogonal electrospray ion source 102b, respectively. In FIG. 1A, the conventional electrospray ion source 102a has a spray needle 104 directed generally towards an inlet 112 of an ion-sampling interface 106. The ion-sampling interface 106 includes a housing 108 defining a lumen 110 wherein the lumen 110 is operable to transport a drying gas 114 past the inlet 112 of the ion-sampling interface 106.
In operation, an electrospray is produced when a sufficient electrical potential difference Vinlet is applied between the inlet 112 of the ion-sampling interface 106 and the fluid at the tip of the spray needle 104 to generate a concentration of electric field lines emanating from the tip of the spray needle 104. When a positive voltage Vinlet is applied at the inlet 112 of the ion-sampling interface 106 relative to the tip of the spray needle 104, the electric field causes negatively-charged ions in the fluid to migrate to the surface of the fluid at the tip of the spray needle 104. Conversely, a negative voltage Vinlet applied at the inlet 112 of the ion-sampling interface 106 relative to the tip of the spray needle 104 will result in positively-charged ions in the fluid migrating to the surface of the fluid at the tip of the spray needle 104. Once the ions are at the surface of the fluid, small charged droplets 116 under the influence of the electric field are urged by electrostatic forces towards the inlet 112 of the ion-sampling interface 106. Solvent rapidly evaporates from the droplets 116, leaving ions 118 from the analyte drawn to and through the inlet 112 of the ion-sampling interface 106 and into the passage of the ion guide. The ions 118 typically are delivered from the ion-sampling interface 106 to a mass spectrometer for analysis.
Conventional electrospray ion sources, such as shown in FIG. 1A, tend to have difficulty with solvent droplets making their way into the vacuum system because the electrosprayed aerosol (droplets 116) exiting from the tip of the spray needle 104 is sprayed directly towards the inlet 112 of the ion-sampling orifice 106. That is, the electrosprayed aerosol 116 exiting from the spray needle 104 and the entry into the vacuum system are located along a common central axis, with the spray needle effluent pointing directly at the entry into the vacuum system and with the spray needle being considered to be located at an angle of zero (0) degrees relative to the common central axis.
In an orthogonal electrospray ion source 102b, such as shown in FIG. 1B, the spray needle 104 is reoriented to a transverse relationship with respect to the ion-sampling interface 106. The transverse orientation allows more efficient enrichment of the analyte ions 118 by spraying the charged droplets 116 in the electrosprayed aerosol past the ion-sampling interface 106, while directing the solvent vapor and solvated droplets 116 in the electrosprayed aerosol away from the ion-sampling interface 106 so that they do not enter the vacuum system.
Although the orthogonal design works well, further improvements are sought.